Many different types of data storage media have been developed to store information. Traditional data storage media, for instance, include magnetic media, optical media, and mechanical media to name a few. Increasing data storage density is a paramount goal in the development of new or improved types of data storage media.
In traditional media, individual bits are stored as distinct mechanical, optical, or magnetic changes on the surface of the media. For this reason, medium surface area generally poses physical limits on data densities of traditional media.
Holographic data storage media can offer higher storage densities than traditional media. In a holographic medium, data is stored throughout the volume of the medium rather than the medium surface. Moreover, data can be superimposed within the same medium volume using multiplexing techniques. For these reasons, theoretical holographic storage densities can approach tens of terabits per cubic centimeter.
In holographic data storage media, entire pages of information, e.g., bit maps, can be stored as optical interference patterns within a photosensitive optical material. This is done by intersecting two coherent laser beams within the optical material. The first laser beam, called the object beam, contains the information to be stored; and the second, called the reference beam, interferes with the object beam to create an interference pattern that can be stored in the holographic recording material as a hologram. In most conventional holographic recording systems, the object beam and reference beam ordinarily follow separate optical paths.
When a stored hologram is illuminated with only the reference beam, some of the reference beam light is diffracted by the hologram interference pattern. Moreover, the diffracted light can be directed to reconstruct the original object beam. Thus, by illuminating a recorded hologram with the reference beam, the data encoded in the object beam can be reconstructed and detected by a data detector such as a camera.
Self-referenced holography as described in U.S. patent application Ser. No. 09/813,066, filed Mar. 20, 2001 for Jathan Edwards entitled “Self-Referenced Holographic Storage,” can improve holographic data storage systems. In self-referenced holography, the object beam and reference beam follow a common optical path. In particular, in self-referenced holography, the reference beam is created from a zero frequency Fourier component of the object beam. As described in U.S. patent application Ser. No. 09/813,066, the creation of the reference beam from the zero frequency Fourier component of the object beam can be achieved using a lens or a mirror. The entire content of U.S. patent application Ser. No. 09/813,066 is incorporated herein by reference.
For example, a lens can be positioned in the optical path of the object beam before the medium to optically direct the zero frequency component of the object beam in order to create a reference beam. Alternatively, a mirror can be positioned in the optical path of the object beam after the medium to optically reflect the zero frequency component of the object beam after it passes through the medium. Generally, self-referenced holography can yield a number of advantages in a holographic data storage system, including the realization of a single optical path directed toward a holographic recording medium which can reduce the size of the system and possibly reduce alignment concerns.